The present invention relates to a method for stretching plastic nets and to the apparatus for performing the method.
The expression "stretching method" designates the modification, by mechanical action (stretching), of the morphological structure of a polymeric material in order to obtain a specific performance, such as an increase in pulling strength and in the elasticity modulus of the product and a decrease in breaking elongation.
It is known that in order to perform the stretching method the material is first heated, directly raising the temperature of its outer surface and then gradually increasing the temperature of its inner layers by conduction.
Systems based on water, air, hot rollers and infrared radiation are used to perform heating. In particular, all currently known stretching systems utilize the principle of heat conductivity to transfer heat from the outer layers to the inner layers of the product.
Accordingly, it is necessary to produce a difference in temperature between the inner layer and the outer layer in order to produce heat exchange and a certain variable amount of time is required to provide a uniform distribution of the temperature inside the material, in order to obtain the intended thermal conditions.
Conventional systems suffer the drawback that they frequently damage the surface of the products subjected to high temperature for a certain time, i.e., for the time required to make the heat uniformly penetrate the inner layers.
The temperature difference between the inner and outer layers causes creep in the overheated outer layers and cracks and fissures in the insufficiently heated inner layers.
Another drawback is further constituted by the intense internal stresses in the presence of sharp temperature gradients caused by the difference in temperature between the outer and inner layers both during heating and during cooling.
These stresses cause breakages or less than optimum alignments during stretching; accordingly, the finished product can be affected by the damage caused by the systems currently being used and can have undesirable cracks, brittleness, elongations and discontinuities.
Another drawback arising from the use of known systems is the large amount of energy required to heat plastic materials with current conduction systems, which often entail problems in dissipating the heat in the heat-producing means.
The above-mentioned drawbacks worsen as the thickness of the material to be heated increases and as the differences in thickness of the cross-sections of the product increase.
Temperature adjustment in known stretching systems also does not allow precise and immediate variations in the temperature of the product to be stretched, owing to the thermal inertia that is typical of the heating system used so far.
Another drawback is also caused by the fact that in order to efficiently vary the temperature of the entire product it is necessary to vary the temperature of the heating means, which in turn propagates, with its specific inertia, the variation to the surface of the net.
In order to try to improve stretching conditions, in known systems the materials are stretched substantially during the heating of the net, since the degree of dissipation of the superficially absorbed heat is high.
In known systems, energy utilization is not strictly proportional to the amount of polymer to be heated, since in any case it is necessary to bring, and keep, the space wherein stretching is performed to a temperature such that the conduction process transfers a sufficient amount of heat to the innermost layers.
Known stretching system have the limitation that it is necessary to adapt the stretching speed to the mass of product to be stretched, since the temperature of the heating means cannot be raised further without severely damaging the net, unless resorting to expensive contrivances which increase the heat exposure time by extending the space traveled inside the heating means.
Water-based heating also entails the need to dry the product and to continuously replace the amount of water that has evaporated and been removed.
Air-based heating entails using large amounts of power in order to keep the temperature constant, so as to contrast thermal dissipations; this system also forces stretching of plastic materials at low speed in order to allow a uniform and useful heat conduction effect, consequently reducing productivity.
Heating with metal rollers which are internally heated by means of diathermic oil entails the problem of poor heat transmissivity, which worsens in uneven materials wherein contact is limited only to the protruding surfaces; uneven heat propagation also causes drawbacks during stretching and consequent defects in the final product.
Heating by infrared radiation has the drawback that it directly heats only the surface layer of the material and does not produce uniform heating per unit mass; infrared radiation also entails the drawback that it is ionizing and as such is dangerous to the operator.
Examples of stretching obtained with conventional heating systems are disclosed in U.S. Pat. No. 4,152,479 and in GB-2035191, GB-2073090 and GB-135901.
Plastic materials have already been subjected, in other applications, to heating by means of electromagnetic waves and more particularly radio frequency waves or microwaves. Many plastics, differently from those used in the present invention, are in fact classified as dielectric materials which have the property that they become polarized because their structure has strong electron bonds. Accordingly, when they are placed in an electromagnetic field which varies with radio frequencies or microwave frequencies, plastic materials such as PET, POM and others undergo continuous polarization changes which can increase the temperature of the material.
Examples of teachings related to these applications arise from JP-6246619 and JP-60220730. Differently from the present invention, these patents do not provide for the heating of polyolefins, which constitute the base polymers used by the present invention. These inventions also use a heating step performed with conventional means, which as mentioned can cause the above-cited drawbacks.
It is also currently known to stretch, with heating, by dielectric loss, only strands or films of polyethylene characterized by very low thicknesses and by a uniform and constant thickness (JP-60173114 and JP-57193513).
The thicknesses, the lack of regions that interconnect the strands and the constant thickness and cross-section deeply differentiate the state of the art from the present invention.
Finally, the materials employed in the present invention, namely polyolefins, were hitherto substantially not capable of being heated by dielectric loss.
Since the ability to heat up is in fact linked to a coefficient, known as loss factor, it can be seen that for polyethylene said factor is between 0.001 and 0.0004, while it is between 0.003 and 0.014 for polypropylene, showing an almost nil heating ability; indeed, these materials are applied inside radiofrequency microwave ovens as supports which indeed do not heat up.